Plural kinds of fuel cells have been developed according to the type of electrolyte. In recent years, there has been a tendency that polymer electrolyte fuel cells (hereinafter referred to as PEFCs) are frequently used. The PEFC includes an MEA (Membrane-Electrode-Assembly) and has a configuration in which main surfaces on both sides of the MEA are exposed to an anode gas containing hydrogen and a cathode gas containing oxygen such as air and the anode gas and the cathode gas are caused to electrochemically react with each other, generating an electric power and heat. To be specific, the following electrochemical reactions occur. Thereby, hydrogen at the anode side is consumed and water is generated as a reaction product at the cathode side.Anode; H2→2H++2e−  (1)Cathode; 2H++(½)O2+2e−→H2O  (2)
The PEFC typically has, as a major body, a fuel cell stack (hereinafter referred to as a stack) formed by stacking unit cells (hereinafter referred to as cells). Typically, 10 to 200 cells are stacked and are sandwiched at both ends of the stacked cells between end plates such that a current collecting plate and an insulating plate are disposed between the associated cell and end plate, and the stacked cells are fastened from both ends by fastening bolts.
The cell has a structure in which the MEA is sandwiched between a pair of flat-plate shaped separator plates, to be precise, an anode separator plate and a cathode separator plate.
The MEA includes a polymer electrolyte membrane having hydrogen ion transmissivity for selectively transporting hydrogen ions, and a pair of electrodes stacked on both surfaces of the polymer electrolyte membrane, namely, an anode and a cathode. Thus, a pair of electrodes are formed on both main surfaces of the MEA. Each of these electrodes includes a catalyst layer comprised of electrically-conductive carbon powder as a major component carrying an electrocatalyst (e.g., metal catalyst such as platinum), and a gas diffusion layer (e.g., carbon paper which has been subjected to water-repellent treatment) which is formed outside the catalyst layer and has gas permeability and electron conductivity. Gas seal members and gaskets are disposed on peripheral regions of the MEA so as to sandwich the polymer electrolyte membrane therebetween. The seal members and the like serve to prevent leakage of the anode gas and the cathode gas flowing within the stack to outside and mixing between them.
The separator plate is made of an electrically-conductive material such as resin containing electrically-conductive carbon or metal and is electrically connected to the electrode of the MEA so as to serve as a part of an electric circuit. An anode gas passage and a cathode gas passage are respectively formed on the both surfaces of the MEA and are each configured to extend to connect an inlet and an outlet on each of the surfaces. Thereby, the anode gas and the cathode gas are supplied through the inlets to the anode and to the cathode, respectively, and generated water and surplus gases are carried away through the outlets to outside. These passages may be provided separately from the separator plates. Nonetheless, typically, the passage grooves are provided on the surfaces of the separator plates and the both surfaces of the MEA are sandwiched between the separator plates so as to be in contact with them. Thus, the separator plates serve to mechanically fasten the MEA, and to connect adjacent MEAs electrically in series.
The anode gas passage and the cathode gas passage are formed so that the entire electrode region of the MEA are exposed to the anode gas passage and the cathode gas passage. Typically, the passages have a serpentine shape.
In a state where the polymer electrolyte membrane is saturated with a moisture, the polymer electrolyte membrane has a lower specific resistance, and serves as electrolyte having hydrogen ion conductivity. For this reason, during the power generation operation of the PEFC, the anode gas and the cathode gas are humidified and supplied. During the power generation operation, hydrogen is oxidized, generating water as a reaction product in the cathode gas passage. The water in the humidified anode gas, the water in the humidified cathode gas, and the water generated through the reaction makes a moisture content of the polymer electrolyte membrane saturated, and are discharged outside the PEFC together with the surplus anode gas and the surplus cathode gas.
Since the electrochemical reaction in the cell is an exothermic reaction, it is necessary to cool the cell so that the inner surface of the cell has a catalytic activity temperature during the power generation operation of the PEFC. In a start-up operation of the PEFC, it is necessary to pre-heat the cell so that the inner surface of the cell has the catalytic activity temperature. In addition, a proper temperature control is required during the power generation operation of the PEFC. If the cell is insufficiently cooled, then the MEA rises in temperature, causing vaporization of the moisture from the polymer electrolyte membrane, so that the membrane becomes dried. As a result, deterioration of the polymer electrolyte membrane progresses, and durability of the cell deteriorates, or electric resistance of the polymer electrolyte membrane increases and thus an electric power output decreases, which is known. On the other hand, if the cell is cooled excessively, then the moisture in the reaction gases flowing in the gas passages is condensed, increasing the amount of water in a liquid state contained in the reaction gases. The water in the liquid state forms liquid droplets which adhere onto at least one of the anode gas passage grooves and the cathode gas passage grooves formed on the separator plates, because of surface tension. If the amount of the liquid droplets is significantly large, then the water adhering onto the interior of the passage grooves impede the flow of the gases, causing flooding to occur. As a result, a reaction area of the electrodes decreases, and performance of the PEFC deteriorates, for example, the electric output becomes unstable, which is known.
Furthermore, by efficiently utilizing the electrochemical reaction heat generated in the cell in outside, i.e., configuring a cogeneration system which includes the PEFC as a major part, heat efficiency of the PEFC can be improved.
For these reasons, a heat transmission medium passage is formed to extend to connect an inlet and an outlet between surfaces of the stacked cells of the PEFC stack to allow a heat transmission medium to flow between the surfaces of the stacked cells. The separator plates are made of a highly heat transmissible material. Typically, the separator plate which has increased in temperature due to the exothermic reaction is caused to exchange heat with the heat transmission medium. Passages for the heat transmission medium are typically formed by providing passage grooves on outer surfaces of the separator plates. Alternatively, another members may be provided between the stacked cells to form the passages.
Patent document 1 discloses a fuel cell system including a gas flow inverting means for inverting a flow of the gas supplied to the stack in such a manner that the gas is introduced from a gas outlet and is discharged from a gas inlet, and a control means for controlling the gas flow inverting means to temporarily invert the flow of the gas supplied to the fuel cell stack. The configuration disclosed in the patent document 1 is capable of suppressing the flooding in the interior of the stack and of preventing reduction of efficiency of the fuel cell system.
Patent document 2 discloses a method of operating the fuel cell for repeatedly inverting a flow direction of the cathode gas or a flow direction of the anode gas in the cell. In the patent document 2, because of such inversion, a current density becomes lower and the amount of generated water becomes smaller in a region where the current density was high and the amount of generated water was large, thereby suppressing an event that the cell gets wet due to the generated water and further preventing the wetting of the cell. This makes it possible to prevent occurrence of a problem that the cathode gas or the anode gas is not easily supplied to the electrode as the degree of the wetting progresses, and thereby the electrochemical reaction does not easily occur.
Patent document 3 discloses a technique for continuously changing a flow direction of a fluid flowing within the fuel cell. Patent document 3 describes that temporal stop of the flow of the fluid can be prevented and thereby reduction of the output of the fuel cell can be inhibited.    Patent document 1: Japanese Laid-Open Patent Application Publication No. 2001-210341    Patent document 2: Japanese Laid-Open Patent Application Publication No. 2003-59515    Patent document 3: Japanese Laid-Open Patent Application Publication No. 2004-79431